Our specific aim is to characterize and compare the routes of migration of Schistosoma manseni, S. japonicum, S. haematobium, and Schistosomatium douthitti exploiting our recently developed radioassay and autoradiographic techniques to locate and quantify 75Se-labeled schistosomula in the tissues of the mouse. In particular, we wish to determine the stages of infection at which schistosomular mortality normally occurs and to identify the organ(s) in which they are arrested and destroyed. We consider the sought-for information absolutely essential to understanding the mechanisms of protective immunity to schistosomiasis because, before we can learn how schistosomula are destroyed or arrested by host immune reaction, we must first determine where and at what stage of infection a substantial proportion of them normally die. Our proposed experiments will proceed in two stages, each occupying about one year. First, we will characterize in detail the distribution of schistosomula in all tissues and at strategic intervals up to 28 days after exposure of mouse skin to 75Se-labeled cercariae. Then, hypotheses raised in connection with these preliminary observations as well as already published hypotheses will be tested in a series of crucial experiments.